1. Field of the Invention
The present invention relates generally to ground anchoring systems, and more specifically, to methods and devices used to drill boreholes in rock strata or other earthen formations for ground anchoring systems.
2. State of the Art
There are various situations where it is critical for safety reasons to maintain the integrity of rock formations or to provide secure anchoring of rock bolts and the like. Such situations may be where earth has been excavated that create a steep inclined wall, tunnelling or in underground mining where the ceiling or roof needs to be secured to prevent a cave-in or even large chunks of rock from falling on workers. In addition, there are situations where the ground is used as an anchoring point to which a cable or other structure in tension must be attached. In such situations, a borehole is drilled and an anchoring system is installed.
In underground mining, a system of roof bolts is used to secure the roof and walls of a mine shaft so that they are self-supporting. According to U.S. law, all underground coal mine entries must be roof bolted. In order to increase the speed by which the roof is bolted, roof bolting machines have been developed. Currently, such roof bolters are hydraulically driven miner-mounted bolting rigs that can be maneuvered in a mine opening and that includes one or more drilling stations for installing roof bolts.
A roof bolting machine works by drilling directly into the rock strata with a rock boring drill bit and inserting either conventional bolts, resin roof bolts or cement grouted roof bolts. These machines use bidirectional type drills that are capable of drilling holes into the rock strata of a depth from about four feet to twelve feet. In addition, the machines are used to insert and, in some applications, tighten and tension the roof bolts that are inserted into the predrilled boreholes.
More modern roof bolting machines are automated to remove the risk of having the operator be exposed to falling rock while the roof bolting procedure is being performed. Such roof bolting machines are operated via remote control from a safer position located away from the unsupported roof area. They use the same technique, however, of drilling a borehole, inserting a resin or cement grout cartridge, inserting a roof bolt and spinning the roof bolt to mix the resin or grout until the resin or grout hardens. The roof bolts may be installed in an untensioned or tensioned state, depending on the particular bolting method being employed.
There are primarily two types of roof bolts used in underground mining. In both instances, boreholes are drilled into the roof and/or walls. Long steel rods are inserted into the boreholes and retained in one of two ways. Point anchor bolts or expansion shell bolts are one type of roof bolt. The anchor bolt is typically between about ¾ to 1 inch in diameter and between about 3 and 12 feet in length. An expansion shell is positioned at the end of the bolt that is inserted into the hole. As the bolt is tightened, the expansion shell expands and causes the bolt to be retained within the hole. These types of bolts are considered temporary because corrosion will reduce the life span of such roof bolts. In addition, because they are only secured by the expansion shell, a layer of closely jointed or soft rock at the expansion shell could allow the expansion shell and the roof bolt to move relative to the hole. This can create a dangerous environment, especially in areas where such rock formations are prevalent.
As such, all underground coal mines in the U.S. use some form of resin or cement grouted roof bolts. One such resin grouted roof bolt is comprised of a length of rebar. The rebar is of a similar size to the anchor bolt previously described, but is not provided with an expansion shell. Rather, after drilling the hole, an elongate tube (cartridge) of resin is inserted into the hole. The rebar is then installed after the resin and spun by the installation drill. This opens the resin cartridge and mixes the resin components. The proximal end of the rebar includes a head that engages a roof plate when fully inserted into the borehole. For tensioning applications, the rebar may include an exposed threaded end for receiving a threaded nut that can be tightened against a roof plate, which in turn is pressed against the roof thus holding the rock strata together. Such tensioning applications usually require a point anchor at the distal end of the rebar. In such applications, an expansion shell system may be used in combination with a resin or cement grout to provide a point anchor at the distal end and to allow tensioning of the roof bolt. In other untensioned applications, the rebar is simply inserted with the resin or cement grout and spun until the resin or grout cures. Such resin or cement grouted rebar is considered a more permanent form of roof support with a potential lifespan in excess of twenty years, since the resin or cement grout help prevent corrosion of the rebar. Long sections of cable have also been employed in place of conventional roof bolts. They are installed in a similar manner to conventional resin or cement grouted roof bolts, but may have significantly longer lengths. Even with the resin or cement hardened around the roof bolt, in some underground mines where the rock strata is unstable, or mostly comprised of closely jointed rock or soft rock, the roof bolt can be relatively easily dislodged from the borehole in which it has been inserted. This may occur even when the bolt is tensioned during the installation process or later and without warning when the result could create a potentially serious safety threat. In such environments, current methods of roof bolt installation do not provide any way to increase the load bearing capability of each roof bolt. In other words, if a roof bolt is imbedded in soft or highly fragmented rock formations, there may be no way to know if the roof bolt is going to hold and there is nothing that can help prevent the roof bolt from failing.
As such there is a significant need in the art to provide a method for installing ground anchors, such as roof bolts, that ensures that the ground anchor will be adequately secured to the ground even in conditions of closely jointed or soft rock. There is also a need to provide such a method for installing ground anchors that does not add any significant amount of time to the anchor installation process. In addition, there is a need in the art to provide a method for installing a ground anchor that is easy to follow and consistently produces the desired result of ensuring that the ground anchor will properly perform even in ground conditions that are not conducive for such anchoring system installations.